On 5/14/25 13:02, wangtao wrote:
>> -----Original Message-----
>> From: Christian König <christian.koe...@amd.com>
>> Sent: Tuesday, May 13, 2025 9:18 PM
>> To: wangtao <tao.wang...@honor.com>; sumit.sem...@linaro.org;
>> benjamin.gaign...@collabora.com; brian.star...@arm.com;
>> jstu...@google.com; tjmerc...@google.com
>> Cc: linux-me...@vger.kernel.org; dri-devel@lists.freedesktop.org; linaro-
>> mm-...@lists.linaro.org; linux-ker...@vger.kernel.org;
>> wangbintian(BintianWang) <bintian.w...@honor.com>; yipengxiang
>> <yipengxi...@honor.com>; <liulu....@honor.com>; <feng....@honor.com>
>> Subject: Re: [PATCH 2/2] dmabuf/heaps: implement
>> DMA_BUF_IOCTL_RW_FILE for system_heap
>>
>> On 5/13/25 14:30, wangtao wrote:
>>>> -----Original Message-----
>>>> From: Christian König <christian.koe...@amd.com>
>>>> Sent: Tuesday, May 13, 2025 7:32 PM
>>>> To: wangtao <tao.wang...@honor.com>; sumit.sem...@linaro.org;
>>>> benjamin.gaign...@collabora.com; brian.star...@arm.com;
>>>> jstu...@google.com; tjmerc...@google.com
>>>> Cc: linux-me...@vger.kernel.org; dri-devel@lists.freedesktop.org;
>>>> linaro- mm-...@lists.linaro.org; linux-ker...@vger.kernel.org;
>>>> wangbintian(BintianWang) <bintian.w...@honor.com>; yipengxiang
>>>> <yipengxi...@honor.com>; <liulu....@honor.com>;
>>>> <feng....@honor.com>
>>>> Subject: Re: [PATCH 2/2] dmabuf/heaps: implement
>>>> DMA_BUF_IOCTL_RW_FILE for system_heap
>>>>
>>>> On 5/13/25 11:28, wangtao wrote:
>>>>> Support direct file I/O operations for system_heap dma-buf objects.
>>>>> Implementation includes:
>>>>> 1. Convert sg_table to bio_vec
>>>>
>>>> That is usually illegal for DMA-bufs.
>>> [wangtao] The term 'convert' is misleading in this context. The appropriate
>> phrasing should be: Construct bio_vec from sg_table.
>>
>> Well it doesn't matter what you call it. Touching the page inside an sg
>> table of
>> a DMA-buf is illegal, we even have code to actively prevent that.
> [wangtao] For a driver using DMA-buf: Don't touch pages in the sg_table. But
> the system heap exporter (sg_table owner) should be allowed to use them.
Good point that might be possible.
> If a driver takes ownership via dma_buf_map_attachment or similar calls, the
> exporter must stop using the sg_table.
> User-space programs should call DMA_BUF_IOCTL_RW_FILE only when the DMA-buf
> is not attached.
> The exporter must check ownership (e.g., ensure no map_dma_buf/vmap is
> active) and block new calls during operations.
> I'll add these checks in patch v2.
>
>>
>> Once more: This approach was already rejected multiple times! Please use
>> udmabuf instead!
>>
>> The hack you came up here is simply not necessary.
> [wangtao] Many people need DMA-buf direct I/O. I tried it 2 years ago. My
> method is simpler, uses less CPU/power, and performs better:
I don't think that this is a valid argument.
> - Speed: 3418 MB/s vs. 2073 MB/s (udmabuf) at 1GHz CPU.
> - udmabuf wastes half its CPU time on __get_user_pages.
> - Creating 32x32MB DMA-bufs + reading 1GB file takes 346 ms vs. 1145 ms for
> udmabuf (10x slower) vs. 1503 ms for DMA-buf normal.
Why would using udmabuf be slower here?
> udmabuf is slightly faster but not enough. Switching to udmabuf is easy for
> small apps but hard in complex systems without major benefits.
Yeah, but your approach here is a rather clear hack. Using udmabuf is much more
cleaner and generally accepted by everybody now.
As far as I can see I have to reject your approach here.
Regards,
Christian.
>>
>> Regards,
>> Christian.
>>
>>
>>> Appreciate your feedback.
>>>>
>>>> Regards,
>>>> Christian.
>>>>
>>>>> 2. Set IOCB_DIRECT when O_DIRECT is supported 3. Invoke
>>>>> vfs_iocb_iter_read()/vfs_iocb_iter_write() for actual I/O
>>>>>
>>>>> Performance metrics (UFS 4.0 device @4GB/s, Arm64 CPU @1GHz):
>>>>>
>>>>> | Metric | 1MB | 8MB | 64MB | 1024MB | 3072MB |
>>>>> |--------------------|-------:|-------:|--------:|---------:|-------
>>>>> |--------------------|--
>>>>> |--------------------|:|
>>>>> | Buffer Read (us) | 1658 | 9028 | 69295 | 1019783 | 2978179 |
>>>>> | Direct Read (us) | 707 | 2647 | 18689 | 299627 | 937758 |
>>>>> | Buffer Rate (MB/s) | 603 | 886 | 924 | 1004 | 1032 |
>>>>> | Direct Rate (MB/s) | 1414 | 3022 | 3425 | 3418 | 3276 |
>>>>>
>>>>> Signed-off-by: wangtao <tao.wang...@honor.com>
>>>>> ---
>>>>> drivers/dma-buf/heaps/system_heap.c | 118
>>>>> ++++++++++++++++++++++++++++
>>>>> 1 file changed, 118 insertions(+)
>>>>>
>>>>> diff --git a/drivers/dma-buf/heaps/system_heap.c
>>>>> b/drivers/dma-buf/heaps/system_heap.c
>>>>> index 26d5dc89ea16..f7b71b9843aa 100644
>>>>> --- a/drivers/dma-buf/heaps/system_heap.c
>>>>> +++ b/drivers/dma-buf/heaps/system_heap.c
>>>>> @@ -20,6 +20,8 @@
>>>>> #include <linux/scatterlist.h>
>>>>> #include <linux/slab.h>
>>>>> #include <linux/vmalloc.h>
>>>>> +#include <linux/bvec.h>
>>>>> +#include <linux/uio.h>
>>>>>
>>>>> static struct dma_heap *sys_heap;
>>>>>
>>>>> @@ -281,6 +283,121 @@ static void system_heap_vunmap(struct
>> dma_buf
>>>> *dmabuf, struct iosys_map *map)
>>>>> iosys_map_clear(map);
>>>>> }
>>>>>
>>>>> +static struct bio_vec *system_heap_init_bvec(struct
>>>> system_heap_buffer *buffer,
>>>>> + size_t offset, size_t len, int *nr_segs) {
>>>>> + struct sg_table *sgt = &buffer->sg_table;
>>>>> + struct scatterlist *sg;
>>>>> + size_t length = 0;
>>>>> + unsigned int i, k = 0;
>>>>> + struct bio_vec *bvec;
>>>>> + size_t sg_left;
>>>>> + size_t sg_offset;
>>>>> + size_t sg_len;
>>>>> +
>>>>> + bvec = kvcalloc(sgt->nents, sizeof(*bvec), GFP_KERNEL);
>>>>> + if (!bvec)
>>>>> + return NULL;
>>>>> +
>>>>> + for_each_sg(sgt->sgl, sg, sgt->nents, i) {
>>>>> + length += sg->length;
>>>>> + if (length <= offset)
>>>>> + continue;
>>>>> +
>>>>> + sg_left = length - offset;
>>>>> + sg_offset = sg->offset + sg->length - sg_left;
>>>>> + sg_len = min(sg_left, len);
>>>>> +
>>>>> + bvec[k].bv_page = sg_page(sg);
>>>>> + bvec[k].bv_len = sg_len;
>>>>> + bvec[k].bv_offset = sg_offset;
>>>>> + k++;
>>>>> +
>>>>> + offset += sg_len;
>>>>> + len -= sg_len;
>>>>> + if (len <= 0)
>>>>> + break;
>>>>> + }
>>>>> +
>>>>> + *nr_segs = k;
>>>>> + return bvec;
>>>>> +}
>>>>> +
>>>>> +static int system_heap_rw_file(struct system_heap_buffer *buffer,
>>>>> +bool
>>>> is_read,
>>>>> + bool direct_io, struct file *filp, loff_t file_offset,
>>>>> + size_t buf_offset, size_t len)
>>>>> +{
>>>>> + struct bio_vec *bvec;
>>>>> + int nr_segs = 0;
>>>>> + struct iov_iter iter;
>>>>> + struct kiocb kiocb;
>>>>> + ssize_t ret = 0;
>>>>> +
>>>>> + if (direct_io) {
>>>>> + if (!(filp->f_mode & FMODE_CAN_ODIRECT))
>>>>> + return -EINVAL;
>>>>> + }
>>>>> +
>>>>> + bvec = system_heap_init_bvec(buffer, buf_offset, len, &nr_segs);
>>>>> + if (!bvec)
>>>>> + return -ENOMEM;
>>>>> +
>>>>> + iov_iter_bvec(&iter, is_read ? ITER_DEST : ITER_SOURCE, bvec,
>>>> nr_segs, len);
>>>>> + init_sync_kiocb(&kiocb, filp);
>>>>> + kiocb.ki_pos = file_offset;
>>>>> + if (direct_io)
>>>>> + kiocb.ki_flags |= IOCB_DIRECT;
>>>>> +
>>>>> + while (kiocb.ki_pos < file_offset + len) {
>>>>> + if (is_read)
>>>>> + ret = vfs_iocb_iter_read(filp, &kiocb, &iter);
>>>>> + else
>>>>> + ret = vfs_iocb_iter_write(filp, &kiocb, &iter);
>>>>> + if (ret <= 0)
>>>>> + break;
>>>>> + }
>>>>> +
>>>>> + kvfree(bvec);
>>>>> + return ret < 0 ? ret : 0;
>>>>> +}
>>>>> +
>>>>> +static int system_heap_dma_buf_rw_file(struct dma_buf *dmabuf,
>>>>> + struct dma_buf_rw_file *back)
>>>>> +{
>>>>> + struct system_heap_buffer *buffer = dmabuf->priv;
>>>>> + int ret = 0;
>>>>> + __u32 op = back->flags & DMA_BUF_RW_FLAGS_OP_MASK;
>>>>> + bool direct_io = back->flags & DMA_BUF_RW_FLAGS_DIRECT;
>>>>> + struct file *filp;
>>>>> +
>>>>> + if (op != DMA_BUF_RW_FLAGS_READ && op !=
>>>> DMA_BUF_RW_FLAGS_WRITE)
>>>>> + return -EINVAL;
>>>>> + if (direct_io) {
>>>>> + if (!PAGE_ALIGNED(back->file_offset) ||
>>>>> + !PAGE_ALIGNED(back->buf_offset) ||
>>>>> + !PAGE_ALIGNED(back->buf_len))
>>>>> + return -EINVAL;
>>>>> + }
>>>>> + if (!back->buf_len || back->buf_len > dmabuf->size ||
>>>>> + back->buf_offset >= dmabuf->size ||
>>>>> + back->buf_offset + back->buf_len > dmabuf->size)
>>>>> + return -EINVAL;
>>>>> + if (back->file_offset + back->buf_len < back->file_offset)
>>>>> + return -EINVAL;
>>>>> +
>>>>> + filp = fget(back->fd);
>>>>> + if (!filp)
>>>>> + return -EBADF;
>>>>> +
>>>>> + mutex_lock(&buffer->lock);
>>>>> + ret = system_heap_rw_file(buffer, op ==
>>>> DMA_BUF_RW_FLAGS_READ, direct_io,
>>>>> + filp, back->file_offset, back->buf_offset, back-
>>>>> buf_len);
>>>>> + mutex_unlock(&buffer->lock);
>>>>> +
>>>>> + fput(filp);
>>>>> + return ret;
>>>>> +}
>>>>> +
>>>>> static void system_heap_dma_buf_release(struct dma_buf *dmabuf) {
>>>>> struct system_heap_buffer *buffer = dmabuf->priv; @@ -308,6
>>>> +425,7
>>>>> @@ static const struct dma_buf_ops system_heap_buf_ops = {
>>>>> .mmap = system_heap_mmap,
>>>>> .vmap = system_heap_vmap,
>>>>> .vunmap = system_heap_vunmap,
>>>>> + .rw_file = system_heap_dma_buf_rw_file,
>>>>> .release = system_heap_dma_buf_release, };
>>>>>
>>>
>
